The present invention relates to a process for the distillation of readily polymerizable, ethylenically unsaturated compounds. More particularly, the present invention relates to a process of distillation whereby the amount of polymerization of readily polymerizable, ethylenically unsaturated compounds is substantially reduced during distillation as compared to certain present methods. Still more particularly, the present invention relates to a process of distillation of readily polymerizable, ethylenically unsaturated compounds whereby the material accumulating in the bottom or reboiler area of the distillation apparatus can be reused as opposed to certain present methods wherein such bottom or reboiler area material is a high pollution waste material. Additionally, the present invention relates to a process of distillation of readily polymerizable, ethylenically unsaturated compounds whereby the rate or throughput for a given distillation apparatus can be increased over the rate at which such distillation apparatus can be operated using present methods. Further the present invention relates to a process of distillation of readily polymerizable, ethylenically unsaturated compounds whereby the distillation product is not tinted by the polymerization inhibitor utilized. Further still, the present invention relates to a process of distillation of readily polymerizable, ethylenically unsaturated compounds whereby the ease of handling of the polymerization inhibitor is substantially increased.
It is known that ethylenically unsaturated compounds such as monomeric styrene and divinylbenzene polymerize readily and that the rate of polymerization increases with increasing temperature. Inasmuch as divinylbenzene and styrene produced by common industrial methods contain reaction products, they must be subjected to separation and purification to be suitable to certain further desirable industrial uses. To prevent polymerization during such separation and purification, various prior art processes employing known polymerization inhibitors such as 4-tert-butylcatechol (TBC) and hydroquinone have been employed. However, vacuum distillation is the preferred method for the separation of unstable organic liquid mixtures and the aforementioned prior art inhibitors are effective only in the presence of oxygen. The partial pressure of oxygen in a vacuum distillation column is too low for such conventional inhibitors to be effective. Another prior art method involves the use of sulfur as the polymerization inhibitor. Although sulfur is effective in the absence of oxygen, it is not as effective as the inhibitor of the present invention and, as an additional undesirable feature, forms a high pollution valueless waste material as a bottoms product.
An example of a prior art method is found in the distillation of vinyl aromatic compounds such as divinylbenzene. In such prior art method, the mixture to be distilled generally is first contacted with the chemical polymerization inhibitor, i.e., sulfur and/or TBC. The resulting mixture containing the chemical polymerization inhibitors is then subjected to distillation conditions in the distillation apparatus. The amount of polymer formed in the distillation apparatus and in the high purity product taken therefrom is substantially higher than desired, and occasionally complete polymerization occurs inside the distillation apparatus. For instance, in the process of distilling crude divinylbenzene (a mixture containing divinylbenzenes, diethylbenzenes and monovinylbenzene) to obtain high purity divinylbenzenes, even when inhibited with sulfur and TBC, a divinylbenzene product is obtained which contains significant quantities of polymer which are difficult to separate from the divinylbenzene and detrimental to the use of such divinylbenzenes. Furthermore, the material which is removed from the bottom or reboiler area of the distillation apparatus is a high pollution sulfurous waste material which must be disposed of.
Other prior art methods utilize certain nitroso compounds as a vacuum distillation polymerization inhibitor. An example of such prior art method is found in the distillation of styrene monomer. Such prior art methods have included the use of such nitroso compound polymerization inhibitors as N, -nitroso phenylhydroxylamine (NPH) and p-nitroso-N,N,-dimethylaniline (NDMA). Although such inhibitors are effective in the absence of oxygen, they have certain disadvantages not possessed by the present invention. For example, NPH is substantially insoluble in hydrocarbons and thus, requires use of special mutual solvents in order to permit its admixture with styrene. NDMA has not been found as effective as such inhibitors as sulfur and thus, has not gained commercial acceptance.
It is now an object of the present invention to provide a new and improved process for the distillation of readily polymerizable ethylenically unsaturated compounds.
Another object of the present invention is to provide a new and improved process for the distillation of readily polymerizable ethylenically unsaturated compounds, which process results in higher recovery of the high purity unsaturated compound and produces less undesirable by-products.
Yet another object of the present invention is to provide a new and improved process for the distillation of readily polymerizable ethylenically unsaturated compounds, which process results in the production of less polymerized material in the distillation apparatus.
Still another object of the present invention is to provide a new and improved process for the distillation of readily polymerizable ethylenically unsaturated compounds, which process enables substantially increased ease of handling due to the liquid nature and ready solubility of the utilized polymerization inhibitor in readily polymerizable ethylenically unsaturated compounds.
A further object of the present invention is to provide a new and improved process for the distillation of readily polymerizable ethylenically unsaturated compounds, which process results in a clear, uncolored distillation product.
A still further object of the present invention is to provide a new and improved process for the distillation of readily polymerizable ethylenically unsaturated compounds, which process does not produce high pollution bottom or reboiler area waste material.
Yet a further object of the present invention is to provide a new and improved process for the distillation of readily polymerizable ethylenically unsaturated compounds, which process enables a given distillation apparatus to be operated at an increased rate of throughput without a reduction in efficiency.
Additional objects will become apparent from the following description of the invention herein disclosed.